This invention relates to radiation curable adhesives for use with thermally-shrinkable films or labels.
This invention is directed to the thermally shrinkable films or labels coated with radiation curable adhesive compositions and to a process for applying such films or labels to containers wherein they are adhered to the container surface.
There has been a trend in the packaging industry, and particularly in the beverage segment of such industry, to replace lithographically preprinted cans with blank cans which can be filled as desired. The labeling of these cans is carried out by applying to the container a printed label formed from a heat shrink film some time soon before or after the filling operation.
This method of labeling is particularly attractive, for example, to breweries which often bottle and package their products under a variety of trade names. By utilizing a thermally shrinkable labeling process, these breweries are no longer required to stock a large inventory of pre-printed containers for each brand and could, instead, stock only unprinted containers together with a supply of the appropriate thermally shrinkable printed film labels. Moreover, the presence of the film label around the container would provide additional protection against the printed containers rubbing together during transport.
Initially these labeling operations were carried out using processes and methods such as described in U.S. Pat. No. 3,822,459 issued Jul. 9, 1974, to Aveberg et al. These earlier processes required the formation of a tube or sleeve of thermally shrinkable film which was then placed over the container and heated in order to shrink the film to conform to the size and shape of the container. Recently Hoffman, in U.S. Pat. Nos. 3,765,991; 3,834,963; 4,108,710; 4,704,173; and 4,844,957, has proposed a process which does not require a preformed sleeve and permits the application of the film directly from a continuous roll of film material onto the container.
Both techniques, but especially the high speed continuous operation such as described by Hoffman, require the use of an adhesive which will form an adequate bond between the container and the label. More specifically, the bond must be such that it will not shrink and separate at the seam during the heat shrinking step. The bond should also form a smooth coating which will not bubble or cause creasing of the film during curing.
Moreover, in many applications, the containers are subjected to heating either during hot filling (e.g., fruit juice processing) or after filling as when a separate pasteurization step is required, these processes put additional stress on the adhesive bond. When used in these processes, conventional adhesives do not have adequate heat strength.
To provide the desirable properties required of an adhesive for use with thermally shrinkable films or labels, the prior art teaches the use of hot melt adhesives. Heretofore, curable adhesives for thermally shrinkable films or labels has not been considered.
We have found that radiation curable adhesive compositions permit satisfactory application of thermally shrinkable films or labels onto containers even at the high speeds involved in continuous processes and where such radiation curable adhesive composition gains instant high heat resistance after curing. The resultant bond is strong, and retains its configuration even after the heat shrinking operation and any additional pasteurization operations so that the xe2x80x9cshoulder seamsxe2x80x9d formed at the ends of the container as well as the seam joint formed at the interface of the leading and trailing edge of the film remain in the desired configuration with no distortion of the label or undesirable exposure of the container at the seam lap.
Thus, the present invention is directed to thermally shrinkable films or labels having coated thereon a radiation curable adhesive and to a process for applying thermally shrinkable films or labels to containers comprising the steps of a) coating at least a portion of a thermally shrinkable but unshrunken film or label segment with a radiation curable adhesive; b) applying the film or label to the longitudinal surface of the container; c) subjecting the radiation curable adhesive to a radiation source to effect curing thereof and, d) subjecting the container to heat to shrink the film or label onto the container so as to permanently affix it thereto.
With some adhesive compositions it is possible, and may be desirable, to reverse steps b) and c). Such adhesives undergo initiation and partial polymerization under UV light, but continue and eventually complete their cure after removal from the UV light source. These adhesives complete their cure some time after exposure in what is commonly known as the dark-cure process. Cationic curing systems typically exhibit dark cure behavior, whereas free-radical curing systems typically do not.
In a preferred embodiment, the radiation curable adhesive is applied to a narrow region of the container or at the leading end of the film segment to secure the leading end of the film to the container, the film wrapped around the container and the film segment secured to the container by overlapping the trailing end, a narrow region of which has been coated with the radiation curable adhesive.
Also disclosed herein are shrink labeled containers wherein the labels are affixed to the container utilizing radiation curable adhesive having an epoxidized block copolymer or a cycloaliphatic epoxide as the base resin. The term xe2x80x9csolid hydrogenated tackifierxe2x80x9d as used herein, is intended to mean any composition which is solid at room temperature and which is useful to impart tack to the radiation curable adhesive composition. ASTM D-1878-61T defines tack as xe2x80x9cthe property of a material which enables it to form a bond of measurable strength immediately on contact with another surfacexe2x80x9d.
The term xe2x80x9cbase resinxe2x80x9d as used herein is intended to mean a polymer which undergoes curing upon UV exposure. Radiation curable adhesives as used herein is intended to mean adhesives that generally comprise one or more of the following:
(a) a base resin, such as epoxidized block copolymer (as described in U.S. Pat. No. 5,516,824 and U.S. Pat. No. 5,776,998); and/or a cycloaliphatic epoxy (such as CYRACURE UVI6110 available from UNION CARBIDE); an olefin (including that having a Carbon-Carbon double bond pendant to the backbone or on endsxe2x80x94such materials may be oligomeric, polymeric or monomeric and the backbone may vary in polarity ranging from aliphatic, urethane, polyester and polyether);
(b) a photoinitiator, the type of which is dependent on the type of chemistry of the base resin e.g. cationic photoinitiator suitable for curing epoxidized block copolymer, cycloaliphatic epoxies, and vinyl ether olefins which includes sulfonium or iodonium salts such as SARCAT CD1010, SARCAT CD 1011 AND SARCAT CD 1012 (available from SARTOMER) (note: SARCAT CD1010 is also available under the trade name CYRACURE UVI 6974 from UNION CARBIDE). For free-radical curing systems such as olefinic or thiolene curing systems the following photoinitiators may be suitable: IRGACURE 651, 184 and 1700 and DAROCUR 1173, available from CIBA-GEIGY; as well as GENOCURE LBP available from RAHN; and ESACURE KIP150 available from SARTOMER. Other examples of photoinitiators which may be used include one or more of the following: Benzophenone, Benzyldimethyl ketal, Isopropylthioxanthone, bis(2,6-dimethoxybenzoyl)(2,4,4-trimethylpentyl) phosphineoxide, 2-hydroxy-2-methyl-1 -phenyl-1 -propanone, Diphenyl(2,4,6-trimethybenzoyl)phosphine oxides, 1-hydroxycyclohexyl phenyl ketone, 2-benzyl-2-(dimethylamino)-1-4-(4-morpholinyl)phenyl-1 -butanone, alpha,alpha.-dimethoxy-alpha-phenylacetophenone, 2,2-diethoxyacetophenone, 2-methyl-1 -4-(methylthio)phenyl-2-(4-morpholinyl)-1 -propanone, 2-hydroxy-1 -4-(hydroxyethoxy)phenyl-2-methyl-1 -propanone;
(c) a tackifier, such as the C5/C9 hydrocarbon resins, synthetic polyterpenes, rosin, rosin esters, natural terpenes, and the like. More particularly, the useful tackifying resins include any compatible resins or mixtures thereof such as natural and modified rosins including gum rosin, wood rosin, tall oil rosin, distilled rosin, hydrogenated rosin, dimerized rosin, and polymerized rosin; glycerol and pentaerythritol esters of natural and modified rosins, including the glycerol ester of pale, wood rosin, the glycerol ester of hydrogenated rosin, the glycerol ester of polymerized rosin, the pentaerythritol ester of hydrogenated rosin, and the phenolic-modified pentaerythritol ester of rosin; copolymers and terpolymers of natural terpenes, such as styrene/terpene and alpha methyl styrene/terpene; polyterpene resins generally resulting from the polymerization of terpene hydrocarbons, such as the bicyclic monoterpene known as pinene, in the presence of Friedel-Crafts catalysts at moderately low temperatures; also included are the hydrogenated polyterpene resins; phenolic modified terpene resins and hydrogenated derivatives thereof such, for example, as the resin product resulting from the condensation, in an acidic medium, of a bicyclic terpene and a phenol; aliphatic petroleum hydrocarbon resins resulting from the polymerization of monomers consisting primarily of olefins and diolefins; hydrogenated aliphatic petroleum hydrocarbon resins; and cyclic petroleum hydrocarbon resins and the hydrogenated derivatives thereof; also included are the cyclic or acyclic C5 resins and aromatic modified acyclic or cyclic resins. Mixtures of two or more of the above described tackifying resins may be required;
(d) a diluent, such as a plasticizing or extending oil including olefin oligomers and low molecular weight polymers as well as vegetable and animal oil and their derivatives. The petroleum derived oils which may be employed are relatively high boiling materials containing only a minor proportion of aromatic hydrocarbons (preferably less than 30% and, more particularly, less than 15% by weight of the oil). Alternatively, the oil may be totally non-aromatic. Suitable oligomers include polypropylenes, polybutenes, hydrogenated polyisoprene, hydrogenated polybutadiene, or the like having average molecular weights between about 350 and about 10,000;
(e) a wax, such as a petroleum derived paraffinic or mycrocrystalline wax (including PACEMAKER 53 available from CITGO) is useful for altering the viscosity, green-strength, reducing tack of the final composition;
(f) a compatible polymer such as a block copolymer including polystyrene-polybutadiene-polystyrene, polystyrene-polyisoprene-polystyrene, poly(alpha-methyl-styrene)-polybutadiene-poly(alpha-methyl-styrene), poly(alpha-methyl-styrene)-polyisoprene-poly(alpha-methyl-styrene), as well as the hydrogenated modifications thereof, e.g. polystyrene-poly(ethylene-butylene)-polystyrene. These copolymers may be prepared by methods taught, for example, in U.S. Pat. Nos. 3,239,478; 3,247,269; 3,700,633; 3,753,936; and 3,932,327. For higher polarity systems, polymers such as polyesters (e.g. DYNAPOL materials available from HULS and sulfonated polyesters (available from EASTMAN under the title AQ series) and acrylic polymers (such as ACRONAL AC205 and ACRONAL AC 258 available from BASF) which are also reactive with free-radical systems and non-reactive acrylics (such as those available from SCHENECTADY CHEMICAL). Other, non-limiting examples of additional materials include the following: SBR random copolymers with low ( less than 20%) or high ( greater than 20%) vinyl contents, available under the tradename DURADENE from FIRESTONE (these high vinyl copolymers are reactive and contribute to the crosslinking of the system); EPDM copolymers which can react into the polymer network via unsaturated sites, and saturated analogs (e.g. EP rubber) that can modify the peel and tack of the adhesive. These are available from EXXON under the trade name VISTALON; butyl rubber, which is a copolymer of isoprene and isobutylene and is available from EXXON CHEMICAL COMPANY under the trade name SB BUTYL; polyisobutylene, available from EXXON CHEMICAL COMPANY under the trade name VISTANEX; and liquid polyisopropylene such as is available from KURARAY INC. under the trade name LIR;
(g) an alcohol-containing co-reactant for cationic-curing systems which is often added to adjust crosslink density, Tg, viscosity, and specific adhesion. Examples include, polyesterpolyols available from STEPAN CHEMICAL COMPANY and from UNION CARBIDE; polyalkylene oxide polyols such as PEG and PPG available from UNION CARBIDE; aliphatic diols such as L-2203 available from SHELL (this is an ethylene butylene diol); and L-1203 an ethylene butylene mono-ol available from SHELL; also useful are polybutadiene polyols available from ATOCHEM; epoxidized polybutadiene polyols for alcohols may also be used; and
(h) other additives known to those skilled in the art. These additives may include, but are not limited to, pigments, fillers, fluorescent additives, flow and leveling additives, wetting agents, surfactants, antifoaming agents, rheology modifiers, stabilizers, and antioxidants. Preferred additives are those which do not have appreciable absorption in the wavelengths of interest.